1. Field of the Invention
The present invention relates to an aluminum alloy material for forging and to a continuous casting process therefor, and more particularly, relates to a technique in which parts such as automobile parts, which are required to have high strength and high toughness, can be mass-produced at low cost.
2. Related Art
Forging of light metals such as aluminum is generally performed by die forging after heating a billet produced by extruding a circular cross section or by casting such as continuous casting, semi-continuous casting, or gravity casting at a predetermined temperature. The forging provides materials of a desired strength and toughness. Production process of an aluminum alloy forged product is substantially as follows. That is to say, hot forging is performed on a billet by reheating it to 350 to 550° C. after performing base metal melting of the alloy mixture, billet continuous casting, billet cutting, surface peeling, and homogenizing treatment, whereby a required product shape is obtained. In such a production process, many steps are required, and unnecessary parts, such as a flash, must be removed as scrap, and therefore, the process results in high cost. The proportion of material cost in the forging cost is high, for example, not less than about 30%. Therefore, the material cost for the forging must be reduced as much as possible in order to yield inexpensive forged products.
Known processes for producing aluminum alloy material for forging are as follows.
(1) DC (Direct Chill) Casting Process or Hot-Top Continuous Casting Process
The surface of a bar of material cast by a continuous casting process is peeled off, and the bar is cut to the required length to obtain the material.
(2) Extrusion Bar or Shape
A bar of material cast by a continuous casting process is hot-extruded with a circular cross section or heteromorphic cross section to obtain the material.
(3) Heteromorphic Continuous Cast Billet
A material having a heteromorphic cross section cast by a continuous casting process is sliced to obtain billets.
(4) Cast Products are Used for Forged Materials (Cast Forging)
A material cast in a near net shape (a shape which is approximate to a product) is lightly forged to obtain the material.
(5) Heat Insulating Mold Continuous Casting Process
A mold has a heat insulating structure, and casting is performed while cooling the mold with cooling water spouted from the lower end of the mold to obtain a billet by a casting process in which contact between the mold and the billet is greatly reduced.
In the process described in (1), a periodic inverse segregation layer called a ripple or laps is formed in periodic contact portions with the mold and in boundaries between a header and the mold. When forging is performed in a state of the surface on which the periodic inverse segregation layer is generated, a rough portion is formed on the surface as damage, and the quality is therefore deteriorated. In order to avoid the deterioration of quality, peeling in which the inverse segregation layer is cut off to remove it is performed. However, the manufacturing cost increases because the cost for additional equipment is necessary. In Hot-Top casting of a gas pressurizing type, control of the gas pressure conditions to obtain a smooth cast surface is complicated, and the apparatus is complicated, thereby increasing the equipment cost.
In the process described in (2), the number of steps is comparatively large, thereby increasing the production cost. Moreover, intergranular corrosion occurs because the surface has a coarse recrystallization structure. Therefore, the strength is decreased as a result of the corrosion. According to Japanese Unexamined Patent Application (KOKAI) Publication No. 7-197216, the fatigue strength and intergranular corrosion characteristics of 6000 system aluminum alloy, which is a heat-treated type of alloy, is increased by providing compressive residual stress. However, the production cost is increased because an additional process for providing the compressive residual stress is required. In Japanese Unexamined Patent Application (KOKAI) Publication No. 7-150312, coarse recrystallization is suppressed by omitting a homogenizing treatment before extruding. However, this proposal does not result in a drastically lower cost by decreasing the number of processes because an additional extruding step is required.
In the process described in (3), the surface quality of a billet is equivalent to that in (3) of the material obtained by the process in (1), and therefore, the process has the same problems as the process in (1). Moreover, in the process in (3), since the cooling structure of the mold is complicated the equipment cost for the apparatus is large, and therefore, the production cost is large.
In the process described in (4), casting molds and dies having shapes corresponding to the finished shapes of the product are required for the casting process and the forging process. Moreover, automation for performing mass production for casting to a complicated shape is difficult. Furthermore, for casting to a complicated shape, it is necessary to increase castability by adding Si. However, the addition of Si decreases forgeability, so that the working ratio after casting is comparatively low. Therefore, the casting structure is liable to remain, and the toughness and strength are lower than those in usual forged products. Moreover, examination for defects in casting, such as cavities is required, and consistent quality cannot be obtained.
In the process described in (5), peeling is not required because a comparatively smooth cast surface can be obtained. However, a segregation layer is formed on a billet surface in continuous casting and is projected to form protrusions, which may be melted in a homogenizing treatment and may result in tucking damage in forging. Therefore, a consistently smooth surface is difficult to obtain.
In addition to the above-mentioned technologies to (1) to (5), an electromagnetic continuous casting process has been proposed as a new technique. However, this technique requires special equipment such as electromagnetic shielding, and the equipment cost thus increases, and it is difficult to produce at a low cost with this technique.
In order to produce inexpensive parts, generally, the material cost must be reduced as much as possible. However, extruding requires many steps, and application thereof is limited to small parts with good production efficiency and cold forging in which a material is forward or backward extruded. When large car parts such as suspensions are produced, the cross section thereof is large and the production cost is high. Therefore, this technique cannot be applied largely selling to cars, and these cars cannot be reduced in weight by using aluminum alloy.
When a billet is produced by the above-mentioned continuous casting or semi-continuous casting, the cost can be relatively low. However, surface defects such as sweating, melting, ripple, and laps occur on the cast surface in a solidification process. Therefore, when the forging is performed without any treatment, the surface defects remain as damage after forging, and satisfactory quality cannot be obtained. Therefore, the production cost increases because peeling must be performed on the surface.
Extrusion bars, shapes, and billets produced by continuous casting have a mill scale surface. Therefore, if the die temperature and preheating in hot working are not exactly controlled, the grain recrystallized from the mill scale is coarse and the strength and elongation will be decreased. In order to avoid this problem, suppressing the coarsening of the recrystallization grains by adding Mn, Cr, Zr is proposed in Japanese Unexamined Patent Application (KOKAI) Publication No. 1-283337, Japanese Unexamined Patent Application (KOKAI) Publication No. 7-145440, and Japanese Unexamined Patent Application (KOKAI) Publication No. 2000-144296. However, the final forging temperature must be higher than the recrystallization temperature so as to suppress coarsening of the grain. In a process of forging in heat for the cost reduction and working temperature easily decrease as the forging is performed, and coarse recrystallization grains are easily formed. As is mentioned in the above, coarsening of the recrystallization grains results in a decrease in strength and elongation. In practice, parts are often used with mill scale other than a test piece supplied for investigating the characteristics. Therefore, many problems actually remain in the coarsening of the recrystallization structure of surface layer.